Mechanically decoupled opto-mechanical connector for flexible optical waveguides embedded and/or attached to a printed circuit board

ABSTRACT

Printed circuit boards that include optical interconnects include a flexible optical waveguide embedded or locally attached to the board having at least one end mechanically decoupled from the board during fabrication that can be fitted with a mechanical connector. Also disclosed are processes for fabricating the circuit board.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 11/877,859 filed on Oct. 24, 2007, theentire contents of which are incorporated herein by reference in itsentirety.

TRADEMARKS

IBM® is a registered trademark of International Business MachinesCorporation, Armonk, N.Y., U.S.A. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to mechanically decoupledopto-mechanical connectors for flexible optical waveguides embeddedand/or attached to a printed circuit board. More particularly, theinvention relates to improvements of an optical interface between thewaveguides in an optical printed circuit board and associatedopto-electronics or passive optics thereon.

2. Description of Background

Printed circuit boards (PCBs) generally include multiple integratedcircuits mounted upon their surfaces. PCBs typically contain multipleconductive and dielectric layers interposed upon each other, andinterlayer conductive paths (referred to as vias), which may extend froman integrated circuit mounted on a surface of the PCB to one or moreconductive layers embedded within the PCB.

The speed and complexity of integrated circuits are increasing rapidly.As the number of components per chip, the number of chips per board, themodulation speed and the degree of integration continue to increase,electrical interconnects are facing fundamental limitations in areassuch as bandwidth, bandwidth density, bandwidth times length product,packaging, and power consumption.

The employment of optical interconnects will be one of the majoralternatives for upgrading the interconnection speed wheneverconventional electrical interconnection fails to provide the requiredbandwidth. However, the introduction of optics and the required opticalconnections into the PCB causes problems due to the substantiallydifferent requirements than those of commonly utilized electricalinterconnects, mechanical connectors, thermal interfaces, and the like.One problem is the proper connection of the waveguides in the board withwaveguides in other boards, with opto-electronic modules on board, andwith test equipment such as fiber bundles. For example, connectionsbetween the opto-electronic subassembly and the PCB would requireelectrical lines with high speed capability to the PCB, an opticalcoupling to the waveguides, mechanical connectors between theopto-electronic subassembly and the PCB as well as a thermal interfaceto the heat sink. Connections between boards would require opticalconnections with precise alignment, electrical lines, and ruggedmechanical connections. Connection between the board and the testequipment would require optical connection with precise alignment andcompatibility to standard fiber bundles. The different connections asnoted above and the specific properties required for the connections,i.e., electrical, thermal (different coefficients of thermal expansionbetween dissimilar materials), and the like as well as the largetolerances in current PCB manufacturing processes, lead to numerouspotential problems. For efficient optical coupling, the alignmentaccuracy of multimode polymer waveguides has to be in the range of 5 to10 micrometers, whereas current PCB tolerances are in the range of about100 micrometers.

Accordingly, there remains a need for improvement of the opticalinterface between an opto-enhanced printed circuit board and associatedelectronics thereon.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantagesare provided through the fabrication of a printed circuit boardincluding optical interconnects comprising a rigid substrate; a flexibleoptical waveguide locally attached to the rigid substrate having atleast one end mechanically decoupled from the rigid substrate such thatthe one end can be flexed away from the rigid substrate and fitted witha mechanical connector; and an optical facet incorporated in themechanical connector enabling an optical coupling to the one endmechanically decoupled from the rigid substrate, wherein the rigidsubstrate comprises a rigid upper board laminated to a stiffener boardto define a stack, wherein the flexible optical waveguide is partiallylaminated to the first stack and is configured to provide fixation ofthe flexible optical waveguide to the stack except at the one endmechanically decoupled from the rigid substrate, and wherein theflexible optical waveguide further comprises alignment markers thereon.

The printed circuit board (PCB) provides for mechanical decoupling ofthe optical connector from a rigid substrate, wherein the electricalconnection between the PCB and the opto-electronic subassembly can berigid or flexible. By way of example, a rigid connection can be anopto-electronic module directly attached with a ceramic carrier that ismounted onto the PCB. A flexible connection is provided that introduceselectrical flex as a connection between the PCB and the opto-electronicmodule. As a result, mechanical stress is minimized and increasedversatility is achieved.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates a sectional view of a printed circuit board includinga flexible optical waveguide that is locally attached to the board andhaving a least one end mechanically decoupled from the rigid substrateand fitted with a mechanical connector;

FIG. 2 illustrates a sectional view of an exemplary flexible opticalwaveguide;

FIG. 3 (A-E) illustrates a process for fabricating the printed circuitboard of FIG. 1.

FIG. 4 (A, B) illustrates bottom-up views of the printed circuit boardafter milling and laser drilling of the alignment features, and afterlaser cut to provide release of the flexible optical waveguide;

FIG. 5 illustrates top-down view of the printed circuit board aftertrench milling, optical facet preparation, and flex contour milling; and

FIG. 6 illustrates a printed circuit board, the backplane, whereby thereleased, flexible optical waveguide is used to provide a bend in thelight path, e.g. a 90° bend. Therefore, an optical connection to thewaveguides on the second printed circuit board, the daughter card, isenabled.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

Disclosed herein is a circuit board having a mechanically decoupledopto-mechanical connector for coupling a flexible optical waveguide thatis releasably disposed thereon to an opto-electronic subassembly ormodule. As will be discussed in greater detail below, the flexibleoptical waveguide includes an end portion that is releasably attached ata position that is close to the optical facet and is equipped with apassively aligned connector. This results in a mechanically decoupledopto-mechanical connector for the waveguide with minimal mechanicalstress. Any type of optical element, e.g., active or passive, can beconnected to the connector.

FIG. 1 illustrates a partial cross-section of a circuit board 10 thatincludes a flexible optical waveguide shown generally at 12 having oneend 14 adapted to be releasably attached and equipped with a passivelyaligned connector 16. This results in a mechanically decoupledopto-mechanical connector for the waveguide 12. Any kind of opticalelement, active or passive, can be connected to this opto-mechanicalconnector.

The flexible optical waveguide end 14 is disposed in close proximity toan optical facet (not shown) and also includes one or more passivealignment features 17, e.g., a copper marker. The passive alignmentfeature 17 is utilized to align a mechanical adapter 19, which acts, incombination with the connector 16, as an opto-mechanical connector ofthe waveguide 12. The printed circuit board (i.e., PCB) includes arelatively rigid upper board 18, which can include various electricallines including all high frequency lines. Exemplary board materialsinclude, without limitation, FR4 (woven glass fiber bundles in a resinmatrix), surface laminated circuitry boards, substrate materials usedfor organic and inorganic carriers, polyimides, LCP, and the like.

A stiffener board 20 is shown attached to the upper board 18 using asuitable adhesive 22. The stiffener board may or may not includeelectrical lines depending on the intended application. The flexibleoptical waveguide 12 is attached to the stiffener board 20 using asuitable adhesive 24 and is supported on a flexible substrate 26 thatpermits flexure of the flexible optical waveguide 12. A lower board 28is attached to the existing stack using a suitable adhesive 27. Anadapter 30, which can be a part of a complex connector, is disposed atthe board edge, which may be rigid or flexible depending on the intendedapplication.

FIG. 2 illustrates in greater detail a cross section of an exemplaryflexible optical waveguide 12, which is first fabricated as anindependent optical layer to enable the desired modularity. The flexiblewaveguide includes a flexible substrate 40, which contains the alignmentmarkers 46, e.g. copper markers. By way of example, the alignmentmarkers 46 can be structured into the copper layer of a laminated resincoated copper foil 42, which is disposed on the flexible substrate.Exemplary flexible materials are polyimides, FR4, thin layer materialscommonly used in PCB manufacturing, e.g., resin coated copper, copper,and the like. Disposed thereon is a patterned core layer 48 encasedwithin cladding layers 44. The flexible optical waveguide is laminatedto the PCB.

FIG. 3 (A-E) illustrates an exemplary process for fabricating theflexible optical connector in the printed circuit board (PCB). Theprocess includes local lamination of the stiffener board 20 to theflexible optical waveguide 12. Local lamination may be provided by astructured adhesive tape, bonding film, acrylate adhesive foil,patterned glue, patterned adhesion inhibitor layer, a sacrificial layeror any combination thereof that permits local release as described. Thestiffener board 20, which may or may not include electrical lines, isused to partially stiffen the flex, e.g., close to the connector area.Optionally, a metallic stop may be used to provide a stop layer on thestiffener board. The upper electrical board 18 is then laminated to theexisting stack containing the flexible optical waveguide and stiffenerboard. The lower board 28 is then laminated to the existing stack. FIGS.3, 4 and 5 illustrate the resulting structure after via formation usinglaser drilling and plating processes along with trench milling, andlaser cutting so as to provide optical facet preparation. This isfollowed by flex contour millings and laser cutting, or the like, topermit flex release of the flexible optical waveguide. Connectors 30 canthen be inserted using the passive alignment features 17 in the flexibleoptical waveguide onto the ends of the flexible optical waveguide andthen fixed. An opening is also milled on the top of the board as shownmore clearly in FIG. 5.

The adapter can be part of a complex connector. The mechanical adapterprovides the alignment features. The MT (mechanical transfer) connectoris a standardized optical connector that provides the alignmentfeatures. In the case of the mechanical transfer, there are two guidingpins (e.g., 700 micrometers in diameter and 4.6 millimeter centerspacing) that are centered with the plane of the optical waveguidearray. Subsequent to the assembly of the passively aligned connector;the flexible optical waveguide is locally (close to the optical facet)released. This results in a mechanically decoupled opto-mechanicalconnector for the waveguides.

Any kind of optical element (active or passive) can be connected to theoptic-mechanical connector. By way of example, opto-electronic modulesmay contain optical transceivers, for example, vertical cavity surfaceemitting lasers (VCSELs) and photodiodes (PDs), which serve to transmitand receive optical signals, respectively. These modules can resideon/in the PCB, adjacent to/integrated with processors, applicationspecific integrated circuits (ASICs) and memory controllers, wheneverdense, high speed optical interconnects are required.

The modularity of the above noted systems given by functional separationof the interfaces provides benefits for designing and the reliability ofoptical interconnects. Modularity also enables individual improvement ofeach element, e.g., PCB, OE-subassembly, optics, thermal interfaces, andthe like. This approach not only simplifies the assembly process of theOE subassemblies to the PCB but it also advantageously providesincreased reliability due to reduced mechanical stress in the interface.Board openings as typically required for opto-electronic subassembly canbe optimized to a smaller footprint and increased design freedom. Thisis especially advantageous for an application where the opto-electronicsubassembly is attached below a carrier and therefore protrudes throughthe board. Accordingly, it is desired to have a relatively high I/Odensity located close to the opto-electronics subassembly. This willserve to maximize the board area available for electrical connectionsand enable a high degree of freedom for the electrical design.

An exemplary use of the proposed mechanically decoupled connector isshown in FIG. 6. FIG. 6 illustrates a printed circuit board 50, thebackplane, whereby the released, flexible optical waveguide 12 is usedto provide a bend in the light path 54, e.g. a 90° bend with respect tothe waveguides in the backplane. Therefore, an optical connection to thewaveguides on the second printed circuit board 52, i.e., daughter card,is enabled. This bending enables an optical coupling to the waveguideson an additional printed circuit board, i.e., the daughter card.

The flow diagrams depicted herein are just examples. There may be manyvariations to these diagrams or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order, or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A printed circuit board including optical interconnects, the printedcircuit board comprising: a rigid substrate; a flexible opticalwaveguide locally attached to the rigid substrate having at least oneend mechanically decoupled from the rigid substrate such that the oneend can be flexed away from the rigid substrate and fitted with amechanical connector; and an optical facet incorporated in themechanical connector enabling an optical coupling to the one endmechanically decoupled from the rigid substrate, wherein the rigidsubstrate comprises a rigid upper board laminated to a stiffener boardto define a stack, wherein the flexible optical waveguide is partiallylaminated to the first stack and is configured to provide fixation ofthe flexible optical waveguide to the stack except at the one endmechanically decoupled from the rigid substrate, and wherein theflexible optical waveguide further comprises alignment markers thereon.2. The printed circuit board of claim 1, wherein the flexible waveguidecomprises a flexible substrate and one or more layers of opticalwaveguides.
 3. The printed circuit board of claim 1, wherein theflexible waveguide is configured to provide a bend in an optical pathand enable a position tolerant connection to an arbitrary orientedcomponent.